With development of Wavelength Division multiplexing (WDM), optical signals of tens to hundreds of wavelengths may be transmitted in a same optical fiber simultaneously in an existing optical communication network. In addition, Reconfigurable Optical Add-Drop Multiplexer (ROADM) based technology facilitates as-needed wavelength configuration in optical communication. Thus, inter-site path of a wavelength in an optical network does not always remain the same. A certain wavelength is not always allocated to the same 2 sites.
In a wavelength Division multiplexing system, industrial research has been done on loading a pilot tone signal for a wavelength for various specific applications. A pilot tone signal sometimes is called a low-frequency dither signal. Loading a pilot tone signal to a signal of a wavelength has little impact on transmission performance. For example, in “A transport network layer based on an optical network elements” jointly published in Journal of Lightwave Technology, 1993 by BT laboratory in United Kingdom, Ericsson in Sweden et al, it is proposed to use a pilot tone signal for power management and wavelength channel confirmation as required by failure management in a wavelength Division multiplexing system. In patent publication No. US 005513029, titled “method and apparatus for monitoring performance of optical transmission systems” applied for by Kim B. Roberts of Nortel company in Canada, 1994, a method for monitoring optical amplifier performance is proposed, where a pilot tone signal with a known modulation depth is monitored for forecast of signal and noise components of an optical amplifier. In addition, in paper No.WeB2.2, titled “signal tracking and performance monitoring in multi-wavelength optical networks” published in ECOC'96 conference, 1996 by Fred Heismann et al of Bell lab, US, a solution for implementing online wavelength route tracking in a Wavelength Division multiplexing network is disclosed, where an optical signal of a wavelength is modulated with a unique pilot tone signal, digital information encoding is performed by Frequency Shift Keying, and the pilot tone signal is monitored at any site in an optical network, such that wavelength route information of the whole network may be learned.
For testing an optical signal-to-noise ratio (OSNR) of an existing optical signal of a high rate such as 40 G, 100 G, or the like, in particular that of a polarization multiplexed optical signal, no interpolation or polarization extinction for conventional OSNR detection can be used, and a new method for OSNR detection is required. In a patent of ALCATEL LUCENT company with publication No. US20120106951A1, titled “In-band optical signal-to-noise ratio measurement”, narrowband optical signals of a center-passband and of an offset-passband are filtered out respectively by a narrowband optical tunable filter. Through the pilot tone and the strength of the two narrowband optical signals, the payload optical signal and a strength of an Amplified Spontaneous Emission (ASE) noise accumulated in an Erbium-doped optical fiber amplifier can be computed, so as to compute the OSNR. However, with the method, OSNR calibration or the like has to be performed taking into account a modulation code type of an optical payload signal, with complicated implementation and a complicated algorithm, to the disadvantage of wide-range flexible use in ROADMs.